1. Technical Field
The present invention relates to a technique for printing by using image data and a technique for generating printing data.
2. Related Art
Known halftone processing with which multi-gradation image data is printed by dot data which has a low number of gradations include an error diffusion method for allocating the density error of a case where a gradation number conversion has been done to the surrounding pixels, and ordered dithering for generating dot data by using a dither mask that has received a threshold value arrangement of favorable dispersibility. In a case where a dither mask to which a blue noise characteristic has been applied is used or where a Bayer-type dither mask is used, then the dot arrangement will have favorable dispersibility, and there will be excellent reproducibility of images with two-dimensional spreading of data having relatively small gradation changes. This ordered dithering also has the advantage of making it possible to control the formation of dots by imparting a specific characteristic to the arrangement of threshold values in the dither mask.
As an example, a technique illustrated in Japanese Laid-Open Patent Application Publication No. 2007-15359 (patent document 1) listed below comprises independently creating a dither mask used for the determination of dot formation during forward movement in a case where bidirectional printing is being carried out, and a dither mask used for the determination of dot formation during return movement. Applying a blue noise characteristic to each of the dither masks also has made it possible to print with little decrease in image quality with respect to the displacement of dot formation positions in the bidirectional printing positions.
The applicants have proposed an approach for taking advantage of both such dithering and an error diffusion method (see Japanese Laid-Open Patent Publication No. 2011-66594 (patent document 2)). With this technique, gradation values of pixels in question are first compared with threshold values constituting a dither mask, and according to the results of comparison, the threshold values used for the error diffusion method are varied and also the threshold values are changed according to the gradation values of the target pixels.